Piezoelectric crystal transducer for controlling fluid flow



United States Patent York Filed Sept. 28, 1956, Ser. No. 612,761 2 Claims. (Cl. 137-625.4)

The present invention relates to a crystal transducer and more particularly to a crystal transducer controlling the flow of fluid in hydraulic systems.

In conventional hydraulic systems the flow of fluid is determined and controlled by the utilization of an electrical network linked to the hydraulic equipment by some type of electromagnetic device such as a proportional solenoid or a torque motor, or other similar well known devices. These conventional electromagnetic devices have been quite useful and successful in their present applications; however, with the increase in pressures and temperatures encountered by hydraulic systems in such uses as high speed aircraft installations and radioactive applications, these presently used electromagnetic drives possess undesirable limitations.

For example, such electromagnetic devices as solenoids and torque motors are characterized by the necessity for large signal power inputs, and an auxiliary power amplifier and have a limited frequency response, high production and maintenance costs, and a complexity of construction. Further, the magnetic fields associated with the operation of solenoids attract small magnetic particles in the fluid and have a tendency to plug air gaps in the hydraulic system to reduce the over-all utility of the system.

The present invention provides the integral use of a crystal transducer with a hydraulic system to control the flow of fluids so as to greatly simplify presently used servo systems and, in addition, to realize a considerable gain in over-all utility with respect to performance and cost. The flow of fluid in hydraulic systems may be controlled directly by the use of a crystal transducer functioning as a controlled flapper valve to control the flow through one or more nozzles or indirectly by utilizing a crystal as a driver for a hydraulic control valve which in turn controls the fluid flow.

The application of a crystal transducer in a conventional hydraulic system will eliminate or reduce the importance of a great number of system disadvantages common with the application of solenoid or torque motors for the control of fluid flow. For example, the signal power level for a crystal transducer, measured in milliwatts, is much less than for a solenoid or torque motor wherein the signal power level for a corresponding installation is in the approximate neighborhood of 7 to 10 watts with a 1% to 2 watts standby. Further, the frequency response for a conventional unit is a maximum resonance frequency of 400 cycles, while the response of a crystal transducer linked to a hydraulic system is approximately 1,000 cycles per second and higher depending on load and mountings.

While presently used solenoids and torque motors require a power amplifier having a high signal level in operative coaction therewith, the need for an amplifier with a crystal transducer would depend on the requirements of the system, and if an amplifier should be required, it could be operative at or near the signal levels of the system. Also, the voltage level of a crystal transducer is much higher than in the conventional systems, and a voltage level with a 150 to 200 volt minimum can be expected. Consequently, the utilization of a crystal transducer to control the flow of fluid will result in a system which will be appreciably lighter in weight and in cost, and simple in construction without the many precision parts required in a system using a solenoid or torque motor.

3,152,612 Patented Oct. 13, 1964 Insofar as the present invention is concerned, the term crystal transducer, as used in the specification and claims, applies to a piezoeelctric element of such well known materials as barium titanate, lead metaniobate, and the like, having the piezoelectric property of changing their shape when an electric potential is applied to appropriate surfaces thereon. Materials such as barium titanate and lead metaniobate are not true crystals like quartz since these are obtained synthetically from a powdered compound, doctored with some additives, compressed into desired shapes and fired for several hours in a hot furnace. However, these materials are preferred over well known piezoelectric materials such as quartz because for a given voltage these elements are more active, that is, they change their shape far more than quartz. Also, materials such as barium titanate, and the like, are commonly referred to as ceramics because they may be oxides, nitrides, carbides, berides, aluminides, and silicides, and the like, fired in a furnace, and if they have piezoelectric properties they are referred to as piezoelectric ceramics.

Actually, barium titanate and lead metaniobate are only a few of some 70 possible piezoelectric ceramic materials. However, these two are presently the most commercially available materials, and lead metaniobate has the especially desirable characteristic of being usable in ambient temperatures of approximately 500 C. or above. Insofar as the present invention is concerned, the term crystal transducer applies to any piezoelectric element which is suitable for controlling fluid flow in hydraulic or pneumatic systems.

An object of the present invention is the provision of a crystal transducer to control the flow of fluid directly or indirectly in hydraulic systems.

Another object is to provide a crystal transducer to control the flow of fluid in hydraulic systems so as to eliminate the use of conventional solenoid or torque motors and thereby reduce the complexity and cost of the systems while greatly increasing the frequency response and general utility, especially at high temperatures.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIGURE 1 is a block diagram of a two-stage servo amplifier incorporating a preferred embodiment of the invention;

FIGURE 2 is a cross-sectional view of the two-stage valve of FIGURE 1, showing a preferred embodiment of the crystal transducer of the present invention;

FIGURE 3 is a cross-sectional view showing the construction of the crystal transducer disclosed in FIGURE 2; and

FIGURE 4 is a perspective view, partly in section, of another embodiment of the present invention.

Referring now to the drawings, there is illustrated in FIGURE 1 a block diagram of a hydraulic servo system 10 of the type disclosed in the United States patent application Serial No. 607,235, filed August 30, 1956 by H. H. Christensen, now US. Patent No. 2,819,030, and assigned to the assignee of the instant application, and which servo system operates to control the attitude or direction of travel of dirigible vehicles. The preferred embodiment it) is provided with an input signal source supplied with reference signals, for example, from an autopilot, altitude stabilizer or other control signal source or combination of sources, not shown. The input signal source energizes a crystal valve amplifier which drives a crystal transducer utilizing the mechanical output of a piezoelectric crystal of such material as barium titanate, lead metaniobate, and

the like, to control the flow of fluid in a two-stage valve for actuating an output piston at a predetermined rate. An output position pick-oil is also provided for transmitting a feedback signal indicating the displacement of the output piston to the crystal valve amplifier which, in turn, is combined with the input signal source for completing the loop of the hydraulic servo system.

FIGURE 2 shows the two-stage valve 12 comprising a housing 14 containing the first or flapper-nozzle pilot control stage and the second stage which is a stem-andsleeve valve operated in the conventional manner. The housing 14 is formed with a transducer chamber 16 containing the crystal transducer 18 which drives the flappernozzle pilot control stage to convert an electrical signal into a valve position and comprises a center metallic flapper 20, serving as the main supporting member, with one end 22 rigidly fixed to the housing. In this manner, the flapper is mounted as a cantilever beam with a free end 24 capable of limited movement. A plurality of thin sheets 26 of a piezoelectric material, such as barium titanate, lead metaniobate, or the like, are bonded to the flapper to form the laminated or composite crystal transducer 18 deflected by application of differential electrical voltages to the piezoelectric material. Briefly, the deflection of the transducer is due to the fact that the piezoelectric materials used for the thin sheets 26 have converse piezoelectric properties, application of an electrical voltage causes a deformation of the crystal or sheet, so that deformation of the sheets 26 cause the flapper 26 to deflect in proportion to the applied voltage.

The dimensions of the transducer 18 are predetermined so that cross-coupling of the impressed stresses is negligible and, in turn, sensitivity to shock is minimized. Also, the sheets 26 are bonded symmetrically on both sides of the flapper 20, so that the symmetry cancels any hysteresis that may be introduced through the electrostrictive effect, which may be defined as a change in dimension independent of the sign of an impressed voltage. As can be surmised, the mechanical force output and electrical impedance of this type of crystal transducer is a function of the material, construction, and degree of polarization, which process is analogous to the initial magnetization required by permanent magnets. Therefore, these parameters can be selected to give a desired mechanical output and a suitable electrical impedance to properly match low-power amplifiers. To clarify the point, the crystal transducer 18 deflects when a voltage is applied across it in a manner similar to the reaction of a bimetallic strip with temperature changes. Thus, the crystal transducer can be used in a hydraulic bridge to replace the conventional electrical solenoids or torque motors.

The predetermined movement of the crystal transducer 18 positions the free end 24 of the flapper 20 between two axially aligned nozzles 28 and 30 of the flapper-nozzle pilot control stage in the two-stage valve 12 to vary the relative flow therethrough. The variation in the flow produces a pressure differential between the nozzles 28 and 30 so as to position valve means illustrated in the form of a conventional spring loaded spool valve 32, to control the flow of fluid to a power piston 34 hydraulically associated therewith.

The flapper-nozzle pilot control stage is provided with fixed orifices 36 and 38, coupled to the nozzles 28 and 30 respectively, which comprise the fixed impedances of the familiar and analogous Wheatstone bridge, while nozzles 28 and 30 provide to variable impedance. The input to the Wheatstone bridge analogy is the pressure drop from fluid supply inlet 40 to a drain conduit 42 from the transducer chamber 16. The output from the bridge is the pressure differential applied to the ends of the second stage spool valve 32 and indicated by the term Pit-Pl. The existence of a pressure differential and its resulting force differential moves the spool valve 32 against centering springs 44 and 46. Accordingly, the flow of fluid through the second stage of the two-stage valve 12 is 4 proportional to the displacement of the spool valve 32 from its neutral position as determined by the reaction of the pressure differential between the nozzles 28 and 3t) reacting against the ends of the spool valve 32.

In the operation of the two-stage system 10, the reference signal at the input energizes the crystal valve amplifier wherein the result of the magnitude and sign difference between the feedback signal and the reference for input signal is transmitted as the output to the crystal transducer. In this manner, the amplifier output controls the deflection both in amplitude and direction, of the crystal transducer 18. Accordingly, the position assumed by the flapper Ztl, integral with the crystal transducer, in response to the signal from the amplifier will control the variable orifice area between the nozzles 28 and 30, as shown in FIGURE 3, in an exaggerated manner for purposes of illustration. A change in variable orifice area in either nozzle 23 or 30 will produce pressure drops across all four impedances of the bridge, namely the fixed impedances, orifices 36 and 38, and the variable impedances, nozzles 28 and 30. The pressure drop across the system will result in a difference in pressure on the ends of the spring loaded spool valve 32. Thus, for a given crystal transducer deflection a relative spool valve position is obtained to selectively port oil to the power piston 34, through either conduit 48 or 50, in such a manner as to reduce the magnitude of a difference between the feedback signal and the input or reference signal to a minimum.

The fluid flow through the two-stage valve is controlled so as to displace the power piston 34 without utilization of complex conventional solenoids or torque motors with their low temperature limitations. In addition to a greater high temperature potential, the crystal transducer 18 requires negligible standby electrical control power. The transducer has no magnetic field, thus avoiding the accumulation of magnetic sludge so common with conventional electromagnetic devices. The external electrostatic ficld of the crystal transducer is at ground potential and hence will have minimum attractive properties for any materials suspended in the hydraulic fluid passing through the transducer chamber 16 to the drain outlet 42. This feature of the present invention alone eliminates a potential source of trouble and thus adds to the overall reliability of the crystal transducer 18 to control the flow of fluid in a hydraulic system, such as the two-stage servo amplifier 10.

Naturally, the design of the crystal transducer 18 will vary with each particular installation encountered so as to eliminate, for example, any vibration difficulties encountered in modern aircraft application. Hence, the design of the transducer 18 will have to be of such structural proportions and configuration that the natural frequency of the transducer falls outside of the vibration environment encountered in the intended aircraft or similar installation. Further, the particular techniques for bonding the thin ceramic sheets 26 to the center metallic flapper 24- will also vary with the specific installation since the operating temperature range will control the particular bonding materials utilized.

FIGURE 4 illustrates an embodiment of the present invention wherein a crystal transducer 52 of similar laminated construction as the crystal transducer 18 in the preferred embodiment, is utilized as a driver for a conventional hydraulic control valve 54. The crystal transducer 52, for purposes of illustration, is mounted on a cantilever mounting 56 of such rigidity as to eliminate vibration difficulties between the transducer and the installation. A conventional three-lands valve stem 58 is slideably mounted within a valve sleeve 60 formed in a hydraulic block 62, so that the valve stem 58 can operatively coact with a plurality of ports, integrally formed in the block 62, to control the flow of fluid therethrough.

The valve stem is provided with an axial extension 64 fixed to the crystal transducer 52 so as to be movable therewith in response to an input signal similar to the reference signal of the preferred embodiment 10. As in the crystal transducer 18, the electrical leads from the crystal valve amplifier are secured between the thin ceramic sheets 26, as shown in FIGURE 3, so that the output signal received from the crystal valve amplifier will actuate the crystal transducer 52 in the desired direction and amplitude. In turn, the transducer slideably displaces the valve stem 58 to control the flow of fluid through the hydraulic valve 54. With reference to the preferred embodiment 10, the hydraulic valve 54 of FIG- URE 4 would correspond to the spool valve 32 porting fluid to the power piston 34 for predetermined actuation thereof. As can be seen from the above description of the modification shown in FIGURE 4, its operation is substantially similar to that of the preferred embodiment in that the flow of fluid through the hydraulic valve 54 is indirectly controlled by the transducer 52 through the intermediate valve stem 58 slideably coacting with the valve sleeve 60 and the ports integral therewith.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that it is intended to cover all changes and modifications of the example of the invention herein chosen for the purposes of the disclosure, which do not constitute departures from the spirit and scope of the invention as set forth in the appended claims.

What is claimed is:

1. A crystal transducer for controlling flow of fluid in a hydraulic system, having a cantilver supported center flapper member, a number of bonded pairs of thin ceramic sheets wherein each one of said bonded pair is bonded to a respective side of said flapper member, an input control signal coupled to said plurality of pairs of thin 6 ceramic sheets to produce a predetermined bending moment on said flapper member, a pair of nozzles facing each other and axially spaced, said flapper member being operative between the facing nozzles so as to reciprocate therebetween in response to the input control signal to vary the flow of fluid.

2. A piezoelectric crystal apparatus for controlling fluid flow in response to low signal power inputs comprising a housing having apertures therein adapted to accommodate fluid flow, a crystal transducer for converting electrical signals into mechanical motion, said crystal transducer comprising a cantilever supported center flapper member, a number of bonded pairs of thin ceramic sheets wherein each one of said bonded pair is bonded to a respective side of said flapper member, and an input electrical control signal coupled to said plurality of pairs of thin ceramic sheets to produce a predetermined bending moment on said flapper member, the arrangement of the flapper member and the housing apertures being such that movement of the flapper member in response to said electrical signal controls said fluid flow.

References Cited in the file of this patent UNITED STATES PATENTS 2,365,738 Williams Dec. 26, 1944 2,461,543 Gunn Feb. 15, 1949 2.722,614 Fryklund Nov. 1, 1955 2,767,689 Moog Oct. 23, 1956 2.800,143 Keller July 23, 1957 2,928,409 Johnson et al Mar. 15, 1960 FOREIGN PATENTS 164,473 Australia Aug. 5, 1955 874,370 France May 4, 1942 

1. A CRYSTAL TRANSDUCER FOR CONTROLLING FLOW OF FLUID IN A HYDRAULIC SYSTEM, HAVING A CANTILVER SUPPORTED CENTER FLAPPER MEMBER, A NUMBER OF BONDED PAIRS OF THIN CERAMIC SHEETS WHEREIN EACH ONE OF SAID BONDED PAIR IS BONDED TO A RESPECTIVE SIDE OF SAID FLAPPER MEMBER, AN INPUT CONTROL SIGNAL COUPLED TO SAID PLURALITY OF PAIRS OF THIN CERAMIC SHEETS TO PRODUCE A PREDETERMINED BENDING MOMENT ON SAID FLAPPER MEMBER, A PAIR OF NOZZLES FACING EACH OTHER AND AXIALLY SPACED, SAID FLAPPER MEMBER BEING OPERATIVE BETWEEN THE FACING NOZZLES SO AS TO RECIPROCATE THEREBETWEEN IN RESPONSE TO THE INPUT CONTROL SIGNAL TO VARY THE FLOW OF FLUID. 